Today, the great challenge for management of schistosomiasis in countries with low to moderate endemicity is to move from morbidity control to eradication. Most efforts see initial reductions in prevalence and intensity followed by low-level transmission that constantly threatens resurgence if neglected. Human immigration, the health of the parasite population, potential drug resistance, climatic conditions and the level of human fecal contamination of surface waters can all contribute to persistence. Eradication or even elimination as a public health problem can be viewed as an effort to produce extinction. While extinction and conservation are subjects where population genetics have made important contributions for management, a population genetics perspective is rarely applied when planning and making decisions about parasite control. In longitudinal treatment studies of rural infections, an analysis of population genetic structure showed that immigration (gene flow) contributes little to persistence, at low prevalence widespread drug administration is not likely to promote drug resistance, persistent infections are unlikely to represent drug resistance, there are no host preferences by demographics and re-infection can be distinguished from persistence. Often considered a rural disease, schistosomiasis is increasingly being identified in large urban areas where 85% of the Brazilian population resides, and elimination of schistosomiasis from an urban area can be a more tractable problem than from a rural area. Urban foci in general consist of lighter infections and thus are nearer to elimination as a starting point. Also, urban sanitation is generally better. Finally, focal elimination is more likely in cities where infection exists in small islands. An active site of S. mansoni transmission and human infection was identified in one neighborhood of Salvador, Bahia, Brazil, a city of 3 million, and infected snails in other neighborhoods. Analysis of population structure suggested even greater spatial fragmentation than in rural sites. How the infection is distributed within communities and how the human, parasite and snail populations change over time have implications for how transmission might be eliminated. This proposal will collect longitudinal demographic, parasite genetic, malacologic and environmental data from urban foci of schistosomiasis transmission in Brazil. Two neighborhoods where the prevalence of schistosomiasis will be surveyed yearly for demographic information and schistosome stool eggs, and those infected will be treated. Local water contact sites will also be surveyed monthly for snail infections, human fecal contamination and climatic conditions. Parasite populations will be genetically characterized and tracked, and the spatial and longitudinal distribution of human fecal contamination and snail populations will be assessed and related to risk. The demographic, environmental and climatic data will provide a context for interpreting changes in parasite population structure. Understanding the response of parasite populations under pressure will guide management strategies toward eradication.